The fact that nuclear reactors are powerful antineutrino sources was realized soon after nuclear reactors became practical; antineutrinos were discovered by Cowan and Reines in 1956 using antineutrinos from the Savannah River reactor. Subsequently, it was pointed out by Mikaelyan in 1978 that antineutrinos can be used to monitor nuclear reactors. This monitoring can be either part of the reactor instrumentation to ensure proper operation of the reactor or it can be used for nuclear non-proliferation safeguards. Antineutrino reactor monitoring has been experimentally demonstrated, but in order to become practical for applications, detectors which can perform at the surface, near nuclear reactors are necessary, preferentially not involving any liquids.
International Atomic Energy Agency (IAEA) safeguards currently are highly personnel intensive, relying extensively on tracking material flow (i.e. the balance between materials received and shipped from any given facility). In order to achieve accuracy and reliability, continuity of knowledge (COK) must be preserved. Maintaining and restoring COK in cases where it has been lost because of equipment malfunctions or operator error puts significant strain on IAEA's technical and personnel infrastructure. Therefore, novel technological approaches which can reduce the required manpower and can gracefully recover from a loss of the COK are highly desirable. The ability to recover from a loss of the COK is particularly useful for countries which are relatively new to the international safeguards regime or have given reason for concern in the past.
Antineutrinos are not directly produced in nuclear fission but result from the subsequent β-decays of the neutron-rich fission fragments. On average there are about 6 antineutrinos emitted per fission and thus, for one gigawatt of thermal power a flux of about 1020 antineutrinos per second is produced. The total number of emitted neutrinos is proportional to the total number of fissions in the reactor, i. e., the reactor power. Moreover, the distribution of fission fragments, and hence their β-decays, are different for different fissile isotopes. Thus, careful antineutrino spectroscopy provides information not only about the total number of fissions, but also about the fission fractions of the various fissile isotopes contained in the core. This allows one to determine the plutonium content and power level of the reactor core in situ with a standoff distance of tens of meters. The practical feasibility of reactor monitoring using antineutrinos has been demonstrated using small, ton-sized detectors both in the former Soviet Union and the United States. In both cases, the detectors were underground with an overburden equivalent to 10-20 meters of water and the detectors were using a liquid organic scintillator. The obvious advantage is that antineutrino reactor monitoring is independent of any operator declaration or previous fuel history. For most conceivable applications in non-proliferation safeguards, the independence from any auxiliary information provided by this technique makes it unique. Also for applications in reactor instrumentation the independence from other methods allows to obtain complimentary information to what can be achieved by existing systems.
The key to detecting the very rare antineutrino interactions in the relatively background rich environment at the Earth's surface, and very near to the core of a nuclear reactor—is to have a distinct event signature that is hard to mimic with the environmental radiation. In current reactor antineutrino experiments, antineutrinos have been detected using liquid scintillator doped with gadolinium (Gd) through the inverse β-decay reactionve+p→e++n.  (1)
The e+ results in a prompt in a prompt energy deposition which is followed by a delayed signal induced by the neutron capture on gadolinium that produces multiple γ-rays. Both the prompt energy deposition and the delayed γ-rays will result in ionization in the scintillator, which in turn will emit light. This light is collected and constitutes the actual signal. The coincidence in space and time between these two events serves as a very robust signature that helps to reduce backgrounds to a manageable level. Nevertheless, these experiments are still located deep underground (from 60 to 1,200-meter water equivalent) and they are well removed from the direct gamma and neutron radiation from the reactor core. The ability to determine the position of the prompt e+ event is limited by photon statistics and the ability to determine the location of the neutron capture is limited by the significant range of the γ-rays (10's of cm) produced in the neutron capture on gadolinium.
Recently a novel concept in antineutrino detector technology has been proposed by the SoLiD collaboration. First, they employ 6Li as neutron capture agentn+6Li→α+8H,  (2)and due to the high mass of the α and 3H particles, these particles deposit all their energy within a sub-mm distance from their production point, resulting in a very well localized neutron capture signature. In principle, 6Li (or most other neutron capture agents) can be dissolved in a liquid scintillator (or solid scintillator), but the electron-equivalent energy deposition would be very low (˜0.5 MeV) and there are many accidental background sources that can produce a similar signature making the neutron tag less effective at rejecting backgrounds. A workaround is so-called pulse shape discrimination (PSD) which exploits the fact that in some scintillators (mostly liquid ones) there is a different light emission time profile (typically in the 10s of nano seconds) for particles with a very high energy deposition density. This PSD is a well demonstrated technique in small detectors and requires relatively large amounts of light and fast (and hence expensive) electronics.
In the SoLiD proposal, the 6Li comes embedded in thin sheets of 6LiF:ZnS(Ag), a commercially available inorganic solid scintillator designed for neutron detection with a very high light yield and a long emission time constant (200 ns). These layers are sandwiched between 50×50×50 mm3 organic plastic scintillator cubes, See FIG. 1A. These cubes serve as antineutrino target and detection medium for the prompt signal. They also act as a neutron moderator reducing the initial neutron energy from a few keV down to thermal energies, which is required for effective neutron capture. The mean light emission timescale for the plastic scintillator is ˜10 ns. Thus, the time structure of the signal will distinguish a neutron from any particle depositing energy in the plastic scintillator, since only highly-ionizing massive particles like the α and 3H can deposit significant energy in the thin layer of 6LiF:ZnS(Ag). This results in a very clean neutron tag. Each cube is optically isolated from the other cubes but not from the 6LiF:ZnS(Ag) layer and, the light is collected and read out using wavelength shifting fibers of about 3×3 mm2 cross section. Each cube is crossed by or more two fibers, running along orthogonal axes and parallel to the 6LiF:ZnS(Ag) sheets, which provide event locations to the precision of a single cube (see FIG. 1B). The small ratio of the cross sections of the cubes to the fibers results in a low light collection efficiency of less than 0.5%, which ultimately limits the energy resolution. These precise event locations for both the prompt signal and the neutron capture signal, combined with the very clean neutron tag results in good signal efficiency and very large background rejection.
The Raghavan optical lattice has been developed for the LENS experiment and an optical prototype is shown in FIG. 1C. The proposed LENS (for Low Energy Neutrino Spectroscopy) detector was designed to study the neutrinos from the Sun. The spatial granularity of LENS is based on the Raghavan Optical Lattice (ROL) design in which the detector volume is optically segmented into cubes. Between the cubes there is a thin layer of material with a refractive index lower than the one inside the cubes. This gives rise to total internal reflection inside the cubes. As a consequence, light produced inside a cube will be guided by total internal reflection along the three axis of the lattice. We call this process channeling and the resulting light is called channeled light. Coincident hits on photomultiplier tubes at the ends of the channels digitally determine the 3-D location of the event to a precision set by the cube size. Each cell is always viewed by a unique combination of 6 photomultiplier tubes. Thus the ROL effectively acts as a 3-D array of nuclear counters with bench top sensitivity.
FIG. 1C shows a mini-prototype demonstrating the light channeling within the scintillation lattice. The Raghavan optical lattice design provides very good segmentation and efficient light collection with a manageable number of electronics channels. If an ROL is constructed out of solid scintillator cubes its constructions is greatly simplified as the thin layer of lower refractive index material is provided by a tiny air gap between the cubes. Also, a ROL can be built with a two-dimensional readout without giving up any precision in event location.
There is a proposed detector using 6LiF:ZnS(Ag) sheets surrounding scintillator bars. This concept achieves spatial segmentation in one dimension by reading the light from each segment separately, which will make scaling to multi-ton detectors very difficult. In the same reference the concept of using a wavelength shifter embedded in the plastic scintillator, in contrast to wavelength shifting fibers has been tested and efficient channeling of light by total internal reflection in the longitudinal direction in the scintillator bar has been observed. Only light generated inside the bar can be channeled by total internal reflection. The light generated in the 6LiF:ZnS(Ag) sheets is however external to the bar. The wavelength shifter inside the bar absorbs the light emitted by the sheet and re-emits its isotropically inside the bar, so that it now can be channeled by total internal reflection.